1. Field of the Invention
The present invention relates generally to signal transmission in a wireless communication system, and in particular, to an apparatus and method for minimizing inter-signal interference in a wireless communication system.
2. Description of the Related Art
In the field of wireless communications, new communication systems have recently been developed and new frequency bands have been allocated. Also, new technologies have emerged to achieve miniaturization and digitization beyond conventional analog wireless communication schemes. The significance of such wireless communication systems is increasing in view of the nature of free propagation of waves in the air.
Due to the feature of a wireless propagation path, a wireless communication system can adopt a variety of signal transmission schemes. Among them, there is a polarization-based signal transmission scheme. Polarization is a property that the direction and magnitude of an oscillated electric field is related to a predetermined direction in electromagnetic propagation. Now a description will be made of a wireless communication system utilizing polarization, starting with electric propagation.
FIG. 1 shows propagation of electric waves in a typical wireless communication system. The phase of electric waves is propagated across space according to the polarization of an antenna. Assuming that the electric field E of the electric waves is propagated in a z-axis direction, the x-y plane electric field E denoted by reference numeral 101 at a predetermined position has an x-axis electric field Ex, a y-axis electric field Ey, and a particular polarization phase θ. The electric field 101 is determined according to the polarization of the antenna, the spatial position of the antenna, and reflection and diffraction in surroundings.
If a receive antenna is polarized with a predetermined phase, it has a maximum reception power when receiving waves polarized with the polarization phase. Therefore, the polarization can be utilized to increase efficiency by distinguishing an intended wave or adjusting the polarization angle of a transmit/receive antenna in the wireless communication system using antennas. Although a random polarization phase is propagated instantaneously due to a phase delay caused by the polarization phase θ of the transmit antenna, and reflection and diffraction from a surrounding object, the average reception power measured for a predetermined period reveals that electric waves are propagated and polarized constantly irrespective of frequency or the velocity of a receiver. Polarization-based signal transmission and reception will be described with reference to FIG. 2.
FIG. 2 shows polarization-based signal transmission and reception in a typical wireless communication system. A Base Station (BS) 200 includes first and second polarization antennas 201 and 203, for polarization-based signal transmission, and Mobile Stations (MSs) 230 and 260 receive signals based on polarization.
The first and second polarization antennas 201 and 203 are perpendicular to each other. The BS 200 communicates with the MSs 230 and 260 based on the above-described polarization property. Thus, the MSs 230 and 260 receive signals from the polarization antennas 201 and 203 of the BS 200 through antennas with predetermined polarization phases.
For example, on the downlink, the polarization antennas 201 and 203 send transmission polarization phases in accordance with the received polarization phases of the MSs 230 and 260. The MSs 230 and 260 have a maximum reception power, for communications with the BS 200.
If the polarization difference between two users is adjusted to be orthogonal, as shown in FIG. 2, each polarization has no effect on the other polarization. Thus, it is possible to provide a service to the two users simultaneously.
This polarization-based signal transmission scheme is efficient to a communication system relying on point-to-point communication or using two terminals with orthogonal reception polarization phases. However, if the communication system services two or more users, transmitted/received signals interfere with one another. The inter-signal interference with MSs will be described with reference to FIG. 3.
FIG. 3 shows the polarization phase of each MS in a typical wireless communication system using polarization. A first MS (MS 1) has a polarization phase 301 and a second MS (MS 2) has a polarization phase 303. When the BS sends a signal with the polarization phase 301 to MS 1, part of the polarized signal acts as an interference component 305 to the polarization phase 303 of MS 2.
A polarization phase varies with the polarization phase of an antenna at an MS and an environment, even if it is the average of polarization phases measured for a long time. As a consequence, there may exist no MSs having mutually orthogonal polarization phases, resulting in interference. Inter-signal interference is one obstacle to signal restitution. Accordingly, a need exists for solving the inter-signal interference problem.